R. S. Day, III and others have been shown that approximatley 20% of human tumor lines and viral transformed lines are hypersensitive to alkylating agents due to an apparent absence of Alkylguanine Alkytransferase. We have not detected any consistent absence of a specific mRNA in these alkylation sensitive, mer-cell lines. We have been able to partially purify the enzyme from human liver. This enzyme removes alkylationdamage at the 0pposition of guanine but not at other sites in DNA. We plan to further characterize this enzyme and purify it to the point where amino acid sequence information can be obtained.